Dirt Four-Link Suspension Tech

The parallel four-link rear suspension is common to Dirt Late Model, Dirt Modified stock cars, and Sprint Cars. The original purpose for using the four-link suspension was to have a suspension system that produced very little, even zero, rear steer as the chassis moved vertically. Racers, being true to their nature decided to experiment with various angles in the four-link and found that zero rear steer was not the most efficient under all circumstances.

2/11A four-link Dirt Late Model rear suspension is designed to have a large range of rear steer in either direction. This full range of adjustability allows the racer the opportunity to make adjustments for the changing conditions that occur on dirt surfaces. The attitude of the car on dry slick tracks can be quite radical and we’ll study why this might work.

Today, we have various schools of thought on where to position the links on a four-link system and many more theories on why. Let’s examine what happens with the various changes and look at the big picture to try to understand what is really happening to our cars. Even though the current trend among top Dirt Late Model racers is to minimize the steering characteristics of the rear suspension, there are times when even these teams must get radical.

Basic Four-Link Designs
We’ll be analyzing two basic designs of the four-link rear suspension, the “standard” four-link, that we’ll call a four-link, where both links are forward of the rearend axle tube, and the “Z” link where the top link is rearward and the bottom link is forward. Both of these designs can be positioned to produce near zero rear steer. The four-link design can be made to produce somewhat more rear steer than the Z-link.

3/11<strong>Example 1:</strong> If the right side of our chassis travels up or down in the turns, we’ll see near zero rear steer by positioning the links as shown (reverse the vertical arrows and the horizontal arrows will be reversed too). The same would be true for the left side. The vertical movement of the chassis will cause the top and bottom of the birdcage to move in different horizontal directions which will result in minimal fore and aft movement of the rearend.

One of the big differences between these systems is the effect of weight jacking and forced loading of the left rear tire with the four-link system. As we introduce more rear steer which moves the LR wheel forward, the angle of the bars is such that the wheel is forced down and in trying to lift the chassis, load is transferred onto the LR tire. This loading is desired by some in order to produce more bite under dry and slick conditions.

If you look at the way the car is situated when the rear is severely steered to the right, the LR tire is pointed more to the middle of the front tires and is driving from the middle of the car. Because of the added loading of this tire, most of the rear weight is on this tire. The RR tire is helping to locate the rear of the car, but does little to drive it off the corners.

4/11<strong>Example 2:</strong> Just as in the Z-link example, movement of the chassis with these settings will produce minimal rear steer in the four-link system.

Both of the suspension types are usually attached to a birdcage that isn’t locked to the rear axle tube and where the rearend is free to rotate. A separate structure is attached to the rearend to control rotational movement of the rearend upon acceleration and braking. This could be a “third” link or pull bar, similar to that used on a three-link suspension, a lift arm that runs forward and is attached well in front of the rearend or a combination of several systems.

If the link brackets were mounted solid to the rear axle tube, then as the car rolled in the turns, there would be a significant amount of binding going on because the birdcages would be moving different amounts and possibly in different directions. The suspension would be trying to twist the rearend as each axle tube would be rotated separately.

5/11Here we see a car that is raised up on the left side, and there appears to be a lot of rear steer as the left rear tire is positioned well forward in the wheel well. This would indicate that the links are positioned for maximum rear steer on this four-link car. The front wheels are not turned because the rear is steering this car around the radius of the turn.

If we change the angles of the links so that one side of the car produces more fore/aft movement at the birdcage, we cause that end of the rearend to move in a direction that will cause the rear of the car to steer away from straight ahead. This is called rear steer and most of what is used for dirt racing is steering to the right.

Under some conditions, rear steer is less desirable, especially on hard and tight dirt tracks that act more like asphalt than dirt. Rear steer on dirt tracks that are slick is not only acceptable, but downright necessary under certain conditions.

Understanding Rear Steer
To even begin to understand how the car will rear steer and to what extent, we first need to completely understand the movement of the chassis and what causes this movement. The chassis mounting points of the four-link and Z-link will move vertically as the car transitions into and out of the turns and even down the straightaway and the amount of movement dictates the degree of rear steer.

6/11<strong>Example 3:</strong> Putting the left side chassis mounts for the links in these positions will cause maximum rear steer as the chassis moves up in the turns. We can dial in various amounts of rear steer by moving the connections to different holes for each or both links.

As a chassis rolls in the turns, three basic things can be happening overall: 1) the left side of the chassis may move up and the right side may move down, 2) the right side may move down and the left side may stay near that static location, and 3) the left side may move up and the right side may remain unchanged. With the same set of suspension link locations at each side, a car may well produce very different rear steer characteristics from each of the three scenarios.

A four-link can be made to produce varying amounts of fore and aft movement of each end of the rear axle in either direction, depending on the combined angles of the links. If we start at a neutral setting for the links, meaning that for a certain range of movement up or down, the axle won’t move fore and aft, let’s see how we can produce axle movement.

7/11As in the four-link sketch, the chassis mounting positions are such that maximum rear steer occurs, moving the left rear forward as the chassis moves up.

If, on a four-link, we move the chassis mount for the bottom link up, then as the chassis moves up, the rear axle will move more forward. On the Z-link, we see the same effect for the bottom link. The opposite is true if the chassis moves down. For both systems, the axle would move to the rear. That is exactly why we need to know which way the chassis is moving at each side of the car under all conditions.

Knowledge of the extent and direction of shock travels will come in handy as we plan out our rear geometry. We can translate shock movement to suspension movement. Using either shock travel indicators or data acquisition will tell us what is really happening. I don’t see widespread use of electronic data gathering on dirt cars, so some mechanical device must be used to help us understand our true movements.

A direct influence on chassis movement in the turns is the J-bar or Panhard bar. If the bar is mounted more parallel to the ground, then it will have little influence on the vertical location of the chassis in the turns and the chassis will move similar to Examples 1 or 2. If there is a lot of angle in the bar with the left end higher than the right end (chassis mount to the left side as is most popular), then as the car turns left, the bar angle will have a jacking effect causing the left side of the bar to want to ride up over the right side of the bar as in Example 3. This movement would raise the entire rear of the car.

8/11Should the right side of the chassis move down in the turns, positioning the links as shown will create rear steer to the right by moving the wheel rearward. We will need to know if the right side of our car is moving down, the left side is moving up, or if there is a combination of both. When we can visualize that properly, we can plan out our rear steer design.

Under those conditions, if the car rolls, and we know it does, and the whole chassis rises up, as we too can visually see, then the right side links may well remain in their static locations producing near zero rear steer at that side. On the other side of the chassis, there will be a combined vertical movement of those links to where the lift associated with the bar angle will be combined with the roll lift so that they can produce a large amount of forward movement of the left rear wheel. This movement pulls that end of the axle forward and the rearend will steer to the right.

Upon acceleration off the corners, the rearend will be driven forward and if the forward ends of the links are higher than the rearends, there’s a further movement of the chassis to a higher level.

If we just look at the way the car steers, we might conclude that this isn’t a very good idea. On asphalt this would produce a very loose car that would be undriveable. But if we look at the whole picture, including the aerodynamics of the body, we start to see why our lap times may be lower by doing this on dirt, especially on a very dry slick racetrack.

9/11In this right-side view of the Z-link, downward chassis movement causes the right rear to move to the rear producing rear steer to the right.

Winged Sprint Cars will generally run at an angle to the direction they are moving through the turns. The tall sides on the wing catch a lot of air and will produce a lateral force that is the opposite of the centrifugal force that tries to take the car to the wall. Long story short, the aero force counteracts the lateral g-force and helps the car go faster through the turns, just like having more tire grip.

The combined effects that raise the whole rear of the car also put the rear spoiler higher into the wind stream and that can produce more aero downforce at the rear. This helps give us more traction to
provide better bite off the corners.

If the track has a lot of grip already, then we need much less rear steer and the associated aero help and so we make changes to our rear links so that less rear steer occurs. We may even benefit from creating opposite rear steer, to the left, to gain more rear traction, just like we do on asphalt. The operative word here is change. We must be willing to make changes and a more thorough understanding of what happens with each change will make it easier to do with better results.

10/11If for each suspension system, the number “1” position produced minimal rear steer or movement of the axle at that side (which it should), then if we move from numbers 1 through 4, we would see more rear steer as the left side of the chassis moved up. The four-link left side view shows how moving the links up through the numbers produces more axle movement by causing the birdcage to move forward at both the top and bottom ends.

What happens at many dirt tracks is that the track is wet and tacky as the day starts out. A car that is jacked up and rear steered to the max just won’t get through the mud as well as one that is more level with all four tires on the ground. We can position the links so that there is very little rear steer for these conditions.

As the track dries out and becomes more slick, we may need more rear steer and rear jacking to get the rear spoiler up into the airstream for more rear downforce and to drive the LR tire into the track. The rear steer has more effect on the angle of the body related to the direction that the car is traveling and an aero side force helps pull the car to the left to keep it from sliding. Putting more angle in the J-bar is warranted now.

Once we fully understand how link angle changes produce rear steer at each side, we can make helpful adjustments at the track as the conditions change. We need to plan out which changes to make and be able to do them fast with little effort. A setup sheet that shows which holes to mount the links to for each set of conditions would help the crew make fast changes. If you don’t want your crew to know why you’re doing things (secrecy is an asset at times), then just number the different sheets and tell them to set the car to “Sheet 3,” period.

11/11The Z-link is a little different in respect to the mounting holes because the top link chassis mounting points must be moved down to create the same effect. This shows how going up in number moves the wheel to the front aiding in producing more rear steer.

Because we need to adjust other parameters on the car for changing conditions, we can include spring rate changes, shock changes, and fifth- and sixth-coil changes as well when we make up the setup sheets. Once we develop our setup sheets, as we race the car, we can tweak the numbers according to the results. The process of dialing in the car to the conditions using our setup sheets may take a few races, but at least we’ll have a plan that takes us in a positive direction.